The present invention relates generally to a wireless communication system design, and more particularly to a multiplexing system for time division duplex (TDD) wireless systems that uses a coax cable for simplifying assembly of remote towers in TDD systems.
A TDD system is designed to transmit and receive on the same frequency in different time slots, the mechanism of which is well known by those skilled in the art. Various literatures have described in theory and applicable detail the TDD systems that are deployed in the telecommunications industry today (e.g., Roger Freeman, “Radio System Design for Telecommunications”, Wiley-Interscience, 2nd Edition, April 1997). TDD systems alternated between transmit or receive on the same frequency while FDD systems simultaneously transmit and receive on different frequencies.
A basic remote tower in TDD systems typically includes tower top device such as transmit and receive amplifiers and a ground device such as a transceiver. An amplifier that is placed far and away from a transceiver of the TDD system is typically called a remote amplifier. For a remote amplifier operating in TDD mode, both DC power and a transmit/receive controlsignal to indicate transmit or receive state must be fed from the transceiver to the amplifier. In transmit state, the transmit remote amplifier must be active and the receive remote amplifier must be inactive. In receive state, the receive remote amplifier must be active and the transmit remote amplifier must be inactive. This transmit/receive control signal signal, in the form of a DC level, is typically sent from the transceiver to the amplifier via a dedicated cable. However, a dedicated cable is expensive to implement and equally costly to maintain.
Alternatively, various designs allow the transmit/receive control signal to be passed wirelessly from the transceiver to the amplifier. However, if not implemented properly, aerial interference may interfere with the wireless link so much so that a high error rate, and a correspondingly costly corrective mechanism, is inevitable.
Alternatively, the DC level of the transmit/receive control signal is known to be injected directly into the RF cable that runs the length between the transceiver and the amplifier. This is accomplished by using a bias tee, which is a signal conditioning element that allows DC and RF signals to be applied to a single communication device. However, this design is problematic because the DC level change is difficult to separate from DC power injected onto the same cable. Changes in the power requirement of the remote amplifier that can cause the DC level at the remote amplifier to shift independent of the transmit/receive control signal.
For the reasons above, it is desirable to design a more reliable system that can be used for sending the transmit/receive controlsignal from the base transceiver station to the tower top amplifier.